In practice, the reduced pressure at the top of the distillation column may change on a short-term basis for a variety of reasons, e.g., due to a disturbance in the top condenser, due to variations in the performance of the vacuum pump, or due to a change in the composition of raw materials when the raw material supply is changed, etc. In this case, the concentrations at the top and at the side offtake, which is optionally present, will change, and the predefined specifications of these products are no longer achieved.
The prior art uses a rigid distillate mass control and a variable reflux control (the setpoint being the distillate withdrawal rate), or a rigid reflux and variable distillate mass control (the setpoint being the reflux rate). This control only responds to the distillate yield at the column head and distributes the mass flows. There is only insufficient response to the thermodynamically caused mass shifts. Boiling point shifts resulting from changes in vacuum lead to changes of the mass flow within the distillation columns. Without manual interference, changes in concentration arise that are represented by means of process analytics. The setpoint concentration is reached again by manual changes of operational parameters.
In addition, the prior art uses a bottom temperature control.
Only after an extended transition time, which may last for up to 5 hours, a new steady state condition is reached in which the overhead product or the side product again has the required specifications. The overhead product then no longer has the too low concentration as during the transition period, and also the side offtake no longer contains fractions of the component intended for the overhead product.
The control of a distillation column by adjusting the reflux ratio at the top as a function of the temperature measured at the top is known. In this method, the reflux ratio is adjusted to one of two preset values as a function of a measured temperature difference of two thermometers (German Auslegeschrift DE 1 059 410 of Jun. 18, 1959, Farbenfabriken Bayer AG). Thus, the controlling is effected as a function of the temperature difference of two thermometers disposed at the top end of the column at a vertical distance rather than as a function of the pressure at the column head. In addition, the reflux ratio is set only in two discrete steps rather than continuously.
Heat exchangers within the head of distillation columns for cooling are known from the prior art, for example, from the German Offenlegungsschriften DE 34 16 519 A1, DE 34 36 021 A1, DE 35 05 590 A1 and DE 35 10 097 A1 (Linde AG, 1984 to 1986).
Also known is a method for distilling/rectifying, preferably oleochemical mixtures, especially mixtures of fatty alcohols, in a column comprising a head and a bottom with or without built-in features such as structured or unstructured packings, packing bodies or trays or the like with a specified pressure loss for predefined liquid and vapor loads of the column.
Such a method is described in the article “Die destillative Aufarbeitung oleoche-mischer Stoffe” by Johannisbauer, Peukert, Skrobek, Henkel-Referate 33/1997, pp. 14-21, issued by the Henkel KGaA, Düsseldorf, 1997. It includes a survey of the state of the practical use of distillation and rectification apparatus in the oleochemical industry at that time, a survey that is still valid on the whole.
What is important in the distillative processing of oleochemical substances is the use of as low temperatures as possible, since such substances have a particularly high thermal sensitivity. Therefore, as low as possible a pressure loss over the column's built-in features is also usually sought, since an increased pressure loss requires high bottom temperatures. Therefore, just in the distillation and rectification of oleochemical substances, regular column packings with their low pressure loss per separation stage are employed.
The technical design of rectification columns is determined by two objects: on the one hand, that as many separation stages as possible are to be realized, and on the other, that the pressure loss and thus the bottom temperatures are to be as low as possible.
The number of separation stages and the pressure loss per column height are usually dependent on the liquid load and, even more, on the vapor load on the columns. As a measure of the vapor load, the “comparable air velocity” has been introduced. It takes the influence of the vapor density on the pressure loss as demanded by the Bernoulli equation into account and, in addition, is clearer than the F factor used in the English-speaking literature.
About 20 years ago, most columns of the oleochemical industry were still equipped with ceramic packing bodies or bubble caps, which at best reached 1 separation stage/m of height and a pressure loss of 5 mbar/m of height for a comparable air velocity of 1.5 m/s. Flat bubble caps and metallic packing bodies already provided a significant improvement, which was utilized essentially to prepare products of higher purity.
Then, the giant leap in rectification technology was achieved in the middle of the 1970's due to the development of the regularly shaped column packings of the company Sulzer. The packings were at first prepared from wire mesh, and later also from thin metal plates. With more than 2 separation stages/m and a pressure loss of <1 mbar/m at a comparable air velocity of 2 m/s, substantially higher throughputs could suddenly be achieved in existing plants, or completely new separation concepts could be realized.
A generally uniform arrangement of the column built-in features, such as bubble caps, packings and the like, is usual in the prior art. Therefore, the pressure also increases in an essentially linear way from column head to column head in accordance with the pressure loss of the built-in features. The modern column built-in features, as described above, have a relatively small pressure loss, so that the pressure in the bottom region is only slightly higher than in the head region. A consequence thereof is an undesirable additional evaporation of high-boiling components, which then also arrive at the distillate to be withdrawn. Another drawback resides in the fact that this fraction of high-boiling components that arrives from the bottom at the region of the column head is cooled down there and, after flowing down to the bottom, again takes up heat, so that this process results in an increased energy consumption.